Chalcogenide Letters Vol. 14, No. 11, November 2017, p. 499 - 510
TRIBOLOGICAL PROPERTIES OF NOVEL Cu/NbSe2 COMPOSITES
REINFORCED WITH REDUCED GRAPHENE OXIDE FILLER
J. F. LIa, B. CHEN
a, Q. SHI
bc, C. LI
a, Y. CHU
d, C. LI
ab*
aSchool of Materials Science and Engineering, Jiangsu University, Key
Laboratory of Tribology of Jiangsu Province, Zhenjiang, 212013, P. R. China bSchool of Mechanical Engineering, Jiangsu University, 301, Xuefu Road,
Zhenjiang, 212013, Jiangsu Province, P. R. China cSchool of Mechanical Engineering, Zhenjiang Vocational Technical College,
Zhenjiang, 212016, P. R. China dSchool of Science, Jiangsu University of Science and Technology, Zhenjiang
212003,China Novel Cu-based composites with niobium selenide (NbSe2) and reduced graphene oxide
(RGO) were sucessfully fabricated using powder metallurgy method (PM). The physical
and tribological behaviors of composites were systematically studied. It was found that
compared with copper, Cu-based composites showed increased hardness and decreased
density. Besides, Cu-based composites with 18wt.% NbSe2 and 2wt.% RGO possessed the
excellent tribological properties among all composites. According to experimental data
analysis, a complete and dense tribo-film was formed on the worn surface, mainly
composing of Fe2O3, CuO, Nb2O5, RGO and NbSe2 thiner sheets exfoliated.
(Received September 19, 2017; Accepted November 27, 2017)
Keyword: Cu-based composites; NbSe2; Reduced graphene oxide; Tribo-film
1. Introduction Metallic matrix composites with excellent mechanical and tribological properties used as
advanced enginering materials are the most promising materials in many industries. Especially, Cu-based composites have always attracted wide attention because of good thermal and electrical conductivities, good corrosion resistance, ease of process and low cost compared with all metallic materials[1-5]. Especially in tribology, the appliacation of copper matrix composites was limited greatly due to low hardness and poor anti-wear properties. Some researchers have reported that high loads or sliding speed led to severe damage to copper and its composites in the process of friction [6-8]. Therefore, it is the primary work to improve the tribological properties of copper and its composites by addition of transition metal dichalcogenides (MX2)or graphite etc. However they also had many drawbacks which led to them not be used in the extremely environment. For example, graphite possesses the excellent lubricity, but its’ poor performance in tribological properties would be experienced when in vacuum or dry environments. MoS2 with exceptional low friction coefficient and wear rate was applied in vacuum circumstance. Furthermore, the hardness of graphite and MoS2 is low, which seriously deteriorates the load-carrying capacity of composites. Therefore, it is vital to develop outstanding tribological properties for Cu-based composites to satisfy the requirements.
Transition metal dichalcogenides NbSe2 has a hexagonal layered structure like MoS2 and graphite, which has excellent self-lubricant property[9]. In the past few years, we have studied the tribological properties of NbSe2 into metal matrix. Tang et al [10] have found that Cu/NbSe2 composites containing appropriate NbSe2 show low electrical resistivity and outstanding tribological behaviors. Chen et al [11], who have reported that the addition of copper coated CTNs
* Corresponding author: [email protected]
500
and NbSe2 in copper matrix would improve electrical property and tribological behavior of Cu-based composites. Zhang et al [12] have investigated that the plastic deformation of the substrate can be effectively decreased by filling NbSe2 so that enhance the antifrction and antiwear behaviors of composites.
Recently, extensive attention is paid to graphene for its properties such as high electrical and thermal conductivity, high modulus, fracture strength[13]. With its excellent properties, the tribological properties of graphene have aslo been explored. The tribological properties of metal matrix composites containing graphene were reported in many previous papers. Xu et al [14] have found that TiAl matrix composites with multilayer graphene can drastically not only enhance the strength, but reduce friction coefficient and wear rate of composites. Zhang et al[15]have investigated that Fe-Ni matrix composites with MoS2/RGO exhibits excellent antifriction properties due to the combined effects of solid lubricants and strengthening phase compared with Fe-Ni matrix. Unfortunately, no relevant reports about the investigation of NbSe2 and graphene used in the Cu-based composites were found.
In this work, two lamellar materials, reduced graphene oxide (RGO) and NbSe2 were prepared, and then were added into Cu matrix. And the corresponding reinforcing and lubricating mechanisms were discussed as well.
2. Experimental details NbSe2 were successfully prepared using solid state sintered technology and the fabrication
process were described [16,17]. Exfoliation of graphite using the modified Hummer's method could synthesize successfully graphene oxide (GO)[18]. 4g graphite powder and 4g NaNO3 were slowly added into 200 ml cooled (0
oC) concentrated H2SO4. The solution was stirred mechanically
in a 1000 ml breaker. 30 g of KMnO4 was slowly added into mixture with stirring, and the reaction lasted for about 90 min. After cooling down to room temperature, the mixture was slowly transferred into cold (0
oC) deionized water, followed by adding 30% H2O2 slowly until the
solution changed into bright yellow. The solid product was centrifuged and washed for several times with 5% HCl solution and distilled water until the pH of the mixture was about 7. The GO nanosheets was obtained by drying in a vacuum oven at 60
oC for 8 h.
Five kinds of composites were prepared by powder metallurgy technique. And Table 1 listed the compositions (denoted as C, CN, CGN1, CGN2 and CGN3). Fig.1 shows the schematic illustration of fabrication process of specimen CGN1, CGN2 and CGN3. The prepared GO nanosheets was added into acetone and sonicated by ultrasonic dispersing technology for over 6h so that formed the homogeneous discretely suspension. As-prepared NbSe2 were added into the aforemention suspension, which be mechanically stirred for 3 h. High-purity copper (50m) powders was added and formed a powder slurry with stirring after the hydrazine hydrate was added slowly. The slurry was dried to form the freeze-dried composite powder by freezing and then kept under vacuum (1Pa) to remove the water.
Finally, the mixed powder was cold-presssed at 500 Mpa and sintered in a nitrogen atmosphere at 800
oC for 2h. As the temperature decreased to room temperature, the surface of
specimens were processed for the following tests and analyses. For comparison, specimen C and CN were fabricated using the above fabrication process.
Fig.1. Schematic illustration of fabrication process of specimen CGN1, CGN2 and CGN3
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2.1.Characterization
The phase composition of as-prepared specimens were identified by X-ray diffractometer
(Bruker-AXS). The morphology of as-prepared powders were analyzed by SEM (JSM-7001F)
equipped with EDS, TEM (JEOL-2010). The morphology and phase compositions of worn surface
of specimens was examined via SEM (S-3400N) equipped with EDS and Laser Raman
Spectrometer (DXR). The elemental states of wear scars were analyzed by XPS (Thermo
ESCALAB 250XL ).
Archimedes’s principle was employed to measure the composite densities according to
ASTMC-20 standard. The hardness of specimens was estimated by an MH-5 Vickers hardness
instrument at the load of 5N and the dwell time of 15s.
Wear tests were conducted on a multi-functional tribometer (UMT-2) at room temperature.
The sintered specimens was used as the disc and the size is 20 mm and 5 mm in diameter and
thickness, respectively. The counterpart was the stainless ball produced from GCr15steel and the
friction diameter was 4mm, and its’ hardness was HRC63. Prior to each test experiment, the
surface of the stainless ball and specimens were processed and cleaned with acetone. The wear
tests were set at the sliding speed of 200 rpm (0.0612 m/s), at the test time of 30 min and an
appiled load of 5 N. The wear rate was expressed as the wear volume divided by the applied load
and sliding distance.Further, all tests were repeated under the same condition at least four times.
3. Results and discussion
3.1. Characterization
The microstructure of NbSe2 and GO are shown in Fig.2. It could be seen that NbSe2
product consisted of a large number of microplates exhibiting a hexagonal structure, which
exhibited the average diameter about 3m and 400 nm in thickness(Fig.2a). The TEM image of
NbSe2 taken from microplates is given in Fig.2b, the results corresponded to the result of NbSe2
possessing a hexagonal structure. In order to study the structure in details, the [0001] zone-axis
HRTEM as well as the SAED pattern of NbSe2 is indicated in Fig.2c-d. The lattice fringe of NbSe2
with a typical hexagonal structure had a spacing of 0.62nm consistent with the theoretical d–
spacing for (002) planes. Besides, the SAED pattern further showed the single crystalline nature of
the hexagonal flake and NbSe2 grew normal to [0001] direction[19]. Fig.3 shows the SEM and
TEM images of GO. It clearly demonstrated that synthesized GO nanosheets, curled and winkled,
were flake structures with some foldings.
Fig.2. SEM image of NbSe2 microplates (a), TEM pattern of NbSe2 microplates (b),
HRTEM pattern of NbSe2(c) and SAED pattern of NbSe2 microplates(d)
a b
c d
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Fig. 3. SEM image of GO (a) ,TEM pattern of GO (b) and SAED pattern of GO nanosheet(c)
The phase constituents of NbSe2 and GO were identified by XRD, and results are illustrated in Fig.4. All observed diffraction peaks of NbSe2 could be indexed to hexagonal NbSe2 phase (JCPDS No.65-7464)with calculated lattice constants of a=3.445Å and c=12.55Å. Other peaks from impurities were not detected, indicating that the NbSe2 was of high purity. GO had two strong diffraction peaks: one was at about 2θ = 9.8°, relating to (001) reflection peak, which was due to the formation of intercalated water moieties and oxygen functionalities groups between the layers of GO [20]. And another mild peak at around 20° could be the characteristic peak (002) plane reflections of graphite from the graphene (JCPDS no.01-0646) [21]. The (002) crystal plane was very broad, suggesting that the sample was very disorderedly along the stacking direction.
Fig.4. XRD patterns of NbSe2(a) and GO(b)
3.2. Phase composition, microstructure, hardness and density
Fig.5 shows the XRD patterns of Cu-based composites. Results that copper were the main
phases in all sintered composites and the NbSe2 phase disappeared. CuxNbSe2 as well as a small
amount of Cu2Se phases were observed, which suggested that most NbSe2 reacted with copper and
transformed into CuxNbSe2 and Cu2Se during the sintering process[19]. However, the diffraction
peaks of RGO could not be detected, which might be the strong peaks of copper covered up the
diffraction peaks of RGO [13].
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Fig.5. XRD patterns of as-prepared C, CN and CGN2
In addition, Fig. 6 further exhibits the microstructure and elemental distribution of
specimen CGN2 composite. It could be seen that the dense microstructure was evenly composed
of three obvious phases: the gray, deep gray as well as dark regions, respectively. According to the
elemental distribution, the deep gray region was rich in Cu,Nb,Se and gray region was rich in Cu
while the dark region was rich in C. Combining the results of XRD and Raman spectra technique,
the deep gray, gray and dark region were Cu, CuxNbSe2/NbSe2 and RGO, respectively. It could
also be noted that the RGO exhibited both agglomerated and dispersed states.
Fig. 6. Microstructure and elemental distribution of specimen CGN2
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Table 1. The chemical compositions, density and micro-hardness of Cu-based composites in mass
Specimen Cu RGO NbSe2 Sintered density
(g/cm3)
hardness
(HV)
C 100 0 0 7.31 75
CN 80 0 20 7.27 103
CGN1 80 1 19 7.18 107
CGN2 80 2 18 7.11 112
CGN3 80 3 17 7.04 104
Table 1 lists the compositions, hardness and sintered density of Cu-based composites.
Specimen C had a density of 7.3g/cm-1
and an average hardness of 75HV. Specimen CN showed a
higher hardness of 103HV than specimen C, suggesting that the hardness of composites was
greatly enhanced by the addition of NbSe2 [10]. The hardness of specimen CGN1, CGN2 and
CGN3 increased compared with specimen CN due to the addition of RGO with the excellent
mechanical properties[13,22]. However, the hardness of specimen CGN3 was lower than that of
specimen CGN2, which might be mainly because excess of RGO led to much agglomeration. The
agglomeration of RGO deteriorated the mechanical properties of composites[23]. The introduction
of low density of RGO or NbSe2 led to the sintered density of composites decreased.
3.3. Tribological properties
Fig.7 presents the variation of friction coefficients and wear rates of Cu-based composites
under dry friction. For Fig.7a, It was clear that the specimen C had the highest friction coefficient
about 0.55 at a unsteady state among all samples. As expected, the friction coefficient of Cu-based
composites sharply decreased from 0.55 to 0.23 with the addition of NbSe2, which were more
stable than that of specimen C, but had a upward trend. This was mainly because of the excellent
lubricity of NbSe2 determined by its laminated structure[10]. In addition, the friction coefficient of
composites were lower and fluctuated more slightly than those of specimen C and CN with the
introduction of RGO. Especially, the friction coefficient of specimen CGN2 showed the lowest and
most stable friction coefficient. The results found that the anti-friction properties of Cu-based
composites were remarkably enhanced by the addition of RGO. However, with the content of
RGO further increasing to 3wt.%, the friction coefficient of speciemn CGN3 tended higher value,
which might be related to the agglomeration of RGO. Furthermore, the changing trend of friction
coefficients were almost the same as wear rates. For Fig.7b, the wear rate of specimen C was about
6.5×10-3
mm3.N
-1m
-1. However, the wear rate of composites (specimen CN) was as low as about
3.5×10-3
mm3.N
-1m
-1 with the introduction of NbSe2, which sharply decreased by 46% compared
with specimen C. The addition of NbSe2 could effectively enhance the wear resistance of
copper-based composites[8]. As a result of the reinforcement effect of RGO, the antiwear
properties of copper-based composite with NbSe2 (specimen CGN1) was further enhanced by
RGO[13]. Besides, specimen CGN2 possessed the lowest wear rate about 0.07×10-3
mm3.N
-1m
-1,
which was almost decreased by more than 98% in comparsion with specimen C. Similarly, as the
RGO increased to 3 wt.%, the wear rate of Cu-based composite sightly increased, which might be
because the excess of RGO ocurred some agglomeration so that deteriorated the hardness of
Cu-based composite, and the reduction of hardness was a key factor in decreasing the wear
resistance[19,24]. That is, filling 2 wt.% RGO led to composites possess the lowest friction
coefficient and wear rate. For the reason why the tribological properties of composites greatly
improved by means of filling NbSe2 and RGO, it could be ascribed to two points. One was NbSe2
and RGO provided good lubricity like MoS2 and graphite. Another was the increase of hardness
decreased the friction coefficient and wear rate[25].
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Fig. 7. Friction coefficients and wear rates of Cu-based composites sliding
against stainless steel ball under dry friction
Fig. 8. Noncontact three-dimensional cross-section images of wear tracks obtained on
surface of specimen C (a), specimen CN (b) and specimen CGN2 (c) under the applied
load of 5 N; (d, e, f) Corresponding cross-section profiles of the wear tracks
To further verity specimen CGN2 possessing excellent tribological properties among all
speciemns. Fig.8 illustrates noncontact three-dimensional cross-section images of wear tracks. It
could be clearly seen that the depth and width of the wear scar for specimen C were about 170 m
and1.8 mm respectively, while specimen CN were about 82 m and 1.2 mm. Particularly, the
depth and width of the wear scar for specimen CGN2 were about 7.5 m and 0.45 mm
respectively and the wear scar in Fig.8c was obviously light and smooth compared with that in
Fig.8a and Fig.8b. Above all, Specimen CGN2 showed the excellent tribological properties due to
the combined effect between NbSe2 and RGO, which was consistent with in Fig.7b.
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Fig.9. SEM micrographs of specimen C, CN and CGN2 (5N load, 6.12cm/s). (a)-(c)
represented the worn surface of C, CN and CGN2, respectively. (I)-(III) represented EDS
result of the worn surface of C,CN and CGN2
3.4 Evaluation of worn surfaces
Fig. 9 shows the worn surfaces of composites after wear test. As can be seen in Fig.9a.
pure copper suffered serious wear evidenced by clear adhesive and serious plastic deformation on
the worn surface, indicating that adhesive wear and serious plastic delamination was the main wear
mechanism, which was consistent with the result of pure copper possessing the highest wear rate.
The worn surface of specimen CN seemed to be milder than that of specimen C. Plastic
deformation and adhesion phenomenon on the worn surfaces of specimen CN were restricted due
to the addition of NbSe2[8,10]. Furthermore, apart with small amount of plastic deformation, slight
delamination was formed on the worn surface of specimen CN, which matched well with lower
friction coeffient and wear rate. For specimen CGN2 (Fig.9c), The adhesion and phastic
deformation were not found on the worn surface, which, exhibiting the most smooth area with the
very shallow grooves, formed the continuous and homogenous tribo-film, which accounted for the
lowest friction coefficient and wear rate. This might be because CuxNbSe2/NbSe2 with low
shearing stress were squeezed out from Cu matrix to form the tribo-film during the sliding process,
which led to a low friction coefficient[26]. And RGO penetrated into the very narrow grooves and
gaps on the tribo-film between the asperities of sliding contact, which could change the direct
contact area of the tribo-film-the asperities of counterpart to the tribo-film-RGO-the asperities of
counterpart. The plowing effect of the asperities of counterpart could be greatly resisted to protect
the tribo-film from further damage due to the presence of RGO nanosheets with high hardness,
which was consistent with some reports[26-28].
a
c
b
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The EDS analysis of the area marked by a rectangle on the worn surface of specimens C, CN and
CGN2 are shown in Fig.9 (I-III). For specimen CGN2, the peaks of C, Nb, Se were detected apart
with Cu peak and the atom ratio of Nb and Se elements was close to 1:2, which indicated the
formation of the tribo-film[26]. However, the low Intensity oxygen and Fe peaks also were found
due to some oxides formation occured on the worn surface. It was believed that the oxides, RGO
and CuxNbSe2/NbSe2 coexisted in the tribo-film, which were responsible for friction reduction.
Specimen CGN2 with excellent tribological properties was chosen for further analysis for
determining the anti-wear mechanism of Cu-based composites. Fig.10 displays Raman spectrum
of the unworn surface and worn surface of specimen CGN2. Few peak of NbSe2 was detected on
the surface of specimen CGN2 before wear test. However, NbSe2, Fe2O3, CuO and RGO were
observed on the tribo-film of specimen CGN2 and the peak of NbSe2 became more and more
obvious after wear test, which demonstrated more NbSe2 was reformed on the worn surface. CuO
and Fe2O3 was formed by oxidation of Cu, and Fe transferred from the counterpart[24,29] due to
the effect of friction heat. NbSe2 was reproduced by the decomposition of CuxNbSe2. The
formation of oxides and reformation of NbSe2 accounted for the reduction of friction coefficient
and wear rate during the rubbing process[26]. Besides, the position of the A1g peak of NbSe2 with
noticeable angular widening of the relative profiles shifted negatively from about 230cm-1
to
215cm-1
after wear test compared with the position of A1g peak of as-prepared NbSe2, which meant
the inplane structure of NbSe2 was greatly broken. Above all, NbSe2 were sheared into thiner
sheets in the process of friction and wear, which was mainly ascribed to NbSe2 with a typical
lamellar structure was easily sheared due to van der Waals force between the layers [9,28].
Fig.10. Raman spectrum of the worn surface of specimen CGN2 before and after
wear test(a) and the worn surface of specimen CN and CGN2(b), NbSe2 (c)
Fig. 11 provides the chemical states of Cu,Nb,Se,C and Fe elements on the worn surface
of specimen CGN2. Fig.11a shows that XPS results of the worn surface of specimen CGN2, the
observed areas found six relatively strong peaks. In Fig.11b, XPS spectra peaks of Cu 2p were
loacated at 953.7eV and 932.9eV, which were associated with CuO and Cu2Se, respectively[19,30].
Similarly, the Nb 3d spectrum could be resolved into three binding energy peaks of 202.3 eV,
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207.4 eV and 210.2eV, which were corresponded to the Nb 3d 5/2, Nb 3d 3/2 and Nb 3d 5/2 chemical
states in NbSe2 and Nb2O5, respectively (Fig.11c). The Se 3d spectrum also could be resolved into
two binding energy peaks of 54.2 eV and 55.2eV, which could be corresponded to Cu2Se and
NbSe2 respectively (Fig.11d). The C 1s spectrum showed four peaks at 284.5 eV, 285.1 eV, 285.9
eV and 287.5 eV, which could be attributed to the carbon atoms in different functional groups:
C=C, C-C, C-O and C=O, respectively (Fig.11e). This could be associated with RGO[31]. Besides,
the peak of Fe 2p around 710 eV and 720 eV, which were assigned to the Fe 2p 3/2 and Fe 2p 1/2
chemical states in Fe2O3 and Fe, respectively (Fig.11f)[30]. The tribo-film mianly consisted of
NbSe2, RGO, Cu-Fe-Nb-oxides and Cu2Se, which was consistant with micro-Raman and EDS
results. It could be concluded that wear process promoted the formation of oxides, and formation
of tribo-film was remarkable improved the friction-reducing and anti-wear properties of Cu-based
composites[24].
Fig.11. XPS spectra on the worn surface of specimen CGN2 after wear test
4. Conclusions
The addition of NbSe2 and RGO decreased the density, but increased hardness of copper.
The strengthing effect of RGO was better than that of NbSe2. Cu-based composites with NbSe2
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and RGO exhibited the excellent tribological properties, especially, Cu-based composites
containing 18wt.%NbSe2 and 2wt.%RGO possessed the best tribological properties among all
other samples. EDS, Raman and XPS analysis suggested that the dense and complete tribo-film,
consisting of Fe2O3, CuO, Nb2O5, RGO and NbSe2 significantly improved the tribological
properties.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China
(51275213, 51302112), the Jiangsu National Nature Science Foundation (BK2011534,
BK2011480), the Scientific and Technological Innovation Plan of Jiangsu Province in China
(Grant Nos. CXLX13_645).
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